simple price calculation structure

calculation-structure.php

<?php

class Price
{
    public function getAmount(): int
    {
        return 0;
    }
}

class FirstPrice extends Price
{
    public function getAmount(): int
    {
        return 1000;
    }
}

class SecondPrice extends FirstPrice
{
    public function getAmount(): int
    {
        return 2000 + parent::getAmount();
    }
}

class ThirdPrice extends SecondPrice
{
    public function getAmount(): int
    {
        return 3000 + parent::getAmount();
    }
}


echo (new ThirdPrice)->getAmount() / 1000;